Set tomlNext from environment

Mostly useful for CLI tools such as tomlv:

	% tomlv unicode.toml
	Error in 'unicode.toml': toml: line 3: expected '.' or '=', but got '€' instead

	% BURNTSUSHI_TOML_110=y tomlv unicode.toml
	(no output, ok!)

parse.go

@@ -2,6 +2,7 @@ package toml
 
 import (
 	"fmt"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -10,7 +11,10 @@ import (
 	"github.com/BurntSushi/toml/internal"
 )
 
-var tomlNext = false
+var tomlNext = func() bool {
+	_, ok := os.LookupEnv("BURNTSUSHI_TOML_110")
+	return ok
+}()
 
 type parser struct {
 	lx         *lexer